This application is related to commonly-assigned U.S. patent application Ser. No. 11/928,038, filed Oct. 30, 2007.
The present invention relates to an exhaust gas recirculation system, and more particularly to a method and system for adjusting turbomachine operation after exhaust gas reenters a portion of the turbomachine.
There is a growing concern over the long-term effects of Nitrogen Oxides (hereinafter NOx) and Carbon Dioxide (hereinafter “CO2”) and Sulfur Oxides (SOx) emissions on the environment. The allowable levels of emissions that may be emitted by a turbomachine, such as a gas turbine are heavily regulated. Operators of turbomachines desire methods of reducing the levels of NOx, CO2 and SOx emitted.
Significant amounts of condensable vapors exist in the exhaust gas stream. These vapors usually contain a variety of constituents such as water, acids, aldehydes, hydrocarbons, sulfur oxides, and chlorine compounds. Left untreated, these constituents will accelerate corrosion and fouling of the internal components if allowed to enter the gas turbine.
Exhaust gas recirculation (EGR) generally involves recirculating a portion of the emitted exhaust through an inlet portion of the turbomachine where it is mixed with the incoming airflow prior to combustion. This process facilitates the removal and sequestration of concentrated CO2, and also reduces the NOx emission levels.
There are a few problems with the currently known EGR systems. When the recirculated exhaust mixes with inlet air (forming an inlet fluid) and enters the turbomachine, the specific heat at constant pressure (Cp) is noticeably different than the Cp of the inlet air. Moreover, the relationship between firing temperature and exhaust temperature is altered proportionally to the change in Cp. When the turbomachine is controlled to a given exhaust temperature, the higher Cp will result in a lower firing temperature at the inlet to the turbine which will severely affect the efficiency and heat rate of the machine.
A turbomachine is generally operated according to a control curve, or similar model which incorporates turbomachine operating data including the exhaust temperature, compressor pressure ratio, and the like. The control of the turbomachine generally relies on relatively fixed compositions of the inlet fluid in order to maintain optimum efficiency and heat rate. The current control methodology generally does not account for the relationship change between firing temperature and exhaust temperature due to the recirculated exhaust.
For the foregoing reasons, there is a need for a method of adjusting the operation of a turbomachine while an EGR system operates. The method should allow for maintaining the desired firing temperature range. The method should also allow for integrating the operation of the EGR system with the adjustment of the firing temperature.